Nucleic Acids

Nucleic acids are polymers of nucleotides – molecules composed of a pentose sugar, a nitrogen-containing base, and a phosphate group.

There are two types of nucleic acids: deoxyribonucleic acid, DNA, and ribonucleic acid, RNA. 

Their chemical structures differ depending on which pentose sugar and nitrogenous bases they contain.

The pentose sugar in RNA is ribose, which has a hydroxyl group attached to carbon-2. The sugar in DNA is deoxyribose, which has only a hydrogen atom but no oxygen at carbon-2.

The nitrogenous base is bonded to carbon-1 and the phosphate at carbon-5. Both RNA and DNA contain the bases adenine, cytosine, and guanine; but DNA has thymine, while RNA has uracil. 

In DNA and RNA, guanine and cytosine form complementary base pairs, linked by three hydrogen bonds. Adenine and thymine form base pairs in DNA while adenine and uracil pair in RNA, both linked together by two hydrogen bonds.

Various DNA or RNA polymerase enzymes catalyze the polymerization of nucleotides.

A phosphodiester bond is formed between a hydroxyl group attached to carbon-3 and the phosphate group attached to carbon-5 of the next nucleotide. This reaction leaves an unattached 5’ end with a free phosphate group and an unattached 3’ end with a free hydroxyl group.

When paired with a complementary strand, the two molecules are antiparallel, meaning the 5’ end of one strand pairs with the 3’ end of the other.

The strands are held together by intermolecular forces, including hydrophobic effects, van der Waals interactions, and the specific hydrogen bonds that form between the nitrogenous bases. DNA is a double helix composed of two polynucleotide chains wound around each other. In contrast, RNA is often found as a single-stranded molecule.

However, RNA can bind to a complementary RNA or DNA. It can also exhibit intra-strand complementary base pairing resulting in different types of RNA secondary structures that have distinct functions within the cell.